1. Field of the Invention
The present invention relates to an optical receiver used in an optical transmission system, and in particular to an optical receiver of a coherent receiver type that does not depend on the polarization state of the signal light.
2. Description of the Related Art
In order to realize a super high-speed optical transmission system of 40 gigabits per second (Gbit/s) or above, a transceiver of a RZ-DQPSK (Return-to-Zero Differential Quadrature Phase Shift Keying) modulation format has been developed. From now on, it is desired to still more improve the optical noise immunity of the RZ-DQPSK transceiver, and to miniaturize the optical variable dispersion compensator that occupies a large size, by for example substituting by strong electrical signal processing. As a means for realizing this, for example it is hoped to adopt a coherent receiving method of for example a homodyne type, an intradyne type, or a heterodyne type, and this is being examined (for example refer to F. Derr, “Coherent optical QPSK intradyne system: Concept and digital receiver realization”, Journal of Lightwave Technology. Vol. 10, No. 9, p. 1290-1296, September 1992). By employing the coherent-type receiver, the optical noise immunity is improved by approximately 3 dB, and compared to delayed direct detection, it is considered that the compensation capability of the wavelength dispersion distortion due to electrical signal processing after photoelectric conversion is markedly increased.
However, in the abovementioned coherent optical reception system, there is an inherent problem in that if the polarization state of the local oscillator light output from the local oscillator light source contained in the optical receiver is orthogonal to the polarization state of the received signal light, it cannot be received. The polarization state of the received signal light propagated on the optical transmission path, continuously changes due to the state of the optical transmission line. Therefore a scheme for solving the above problem is important.
As a conventional technique for overcoming the polarization dependency of the coherent optical receiver, for example there is known methods such as those shown next (for example refer to L. G. Kazovsky, “Phase- and polarization-diversity coherent optical techniques”, Journal of Lightwave Technology, Vol. 7, No. 2, p. 279-292, February 1989, and A. D. Kersey et al., “New polarisation-insensitive detection technique for coherent optical fibre heterodyne communications”, Electronics Letters, Vol. 23, p. 924-926, Aug. 27, 1987.).
(I) A method which employs an endlessly tracking automatic polarization controller that enables a control so as to have the polarization state of continually receiving signal light and that of the local oscillator light close to each other.
(II) A method that uses a polarization diversity light reception front end in which a phase hybrid circuit and a photoelectric conversion section are duplicated.
(III) A method that employs polarization multiplexed light as the local oscillator, where the local oscillator light which has mutually orthogonal polarization components and the optical frequency of one of the polarization components is shifted to approximately two times or more than a signal band width, and performs coherent reception and detection for each of the polarization components by performing signal separation in frequency domain after photoelectric conversion.
However, in the above such conventional techniques, there is a problem in that it is difficult to realize coherent optical receivers that are small size, polarization independent, and capable of receiving very high-speed modulated signal light such as 40 Gbit/s. That is to say, to realize the above-mentioned method of (I), an endlessly tracking automatic polarization controller is necessary, and hence miniaturization is difficult. Furthermore, to realize the above-mentioned method of (II), a large scale light receiving front end circuit of more than twice the size is necessary, and hence miniaturization is difficult. Moreover, to realize the abovementioned method of (III), an electronic circuit having a wide band light receiving band of more than thrice the size with respect to the signal band width is necessary, and hence it is difficult to deal with signal light of a high bit rate.
Here the problem of the abovementioned method of (III) is specifically described.
FIG. 8 is a diagram showing a configuration of a coherent optical receiver to which the method of (III) is applied. In this conventional coherent optical receiver, in a local oscillator light generating section 101, light of an optical angular frequency ωL output from a light source 111 is applied to a polarization beam splitter (PBS) 113 via an optical isolator 112, and separated into orthogonal polarization components. Then, one of the polarization components is input to an acousto-optic modulator (AOM) 114 and the optical angular frequency is shifted by ωO. This polarization component with an optical angular frequency ωL+O, and the other polarization component which is separated by the PBS 113, are then combined in a polarization beam combiner (PBC) 115. As a result, for example as shown in the concept diagram of FIG. 9, a local oscillator light ELO for which the polarization component of the optical angular frequency ωL (Ex(t) component in the figure) and the polarization component of the optical angular frequency ωL+ωO orthogonal to this (Ey(t) component in the figure) are polarization multiplexed is generated.
The local oscillator light ELO output from the local oscillator light generating section 101 is combined with the received signal light ES having an optical angular frequency ωS in a multiplexer 102, and then received by a photodetector 103 and converted into an electrical signal. This electrical signal includes a signal component A1 of an intermediate frequency ωi due to the beat of the polarization component of the optical angular frequency ωL included in the local oscillator light ELO, and the received signal light ES, and a signal A2 of an intermediate frequency ωi+ωO due to the beat of the polarization component of the optical angular frequency ωL+ωO included in the local oscillator light ELO, and the received signal light ES. Therefore by applying the output signal of the photodetector 103 to each of the bandpass filters (BPF) 104 and 105, the respective intermediate frequency signals A1 and A2 are separated corresponding to the frequencies. Then, by inputting the respective intermediate frequency signals A1 and A2 to a reception electronic circuit 106, and executing necessary signal processing, the received data DATA is regenerated.
At this time, the intermediate frequency signals A1 and A2 input to the reception electronic circuit 106 come to have an electric spectrum such as shown for example in the schematic view of FIG. 10. More specifically, the intermediate frequency signal A1 has a spectral width of approximately 2 times the signal band width centered in the frequency ωi, and the intermediate frequency signal A2 has a spectral width of approximately 2 times the signal band width centered on the frequency ωi+ωO. Furthermore, a difference ΔP of the power of the intermediate frequency signals A1 and A2 changes depending on the polarization state and the like of the received signal light. Therefore, the band width of the reception electronic circuit 106, in the example of FIG. 10, must be 4 times or more the signal band width. In the case where the optical angular frequency ω of the local oscillator light is set so that the intermediate frequency ωi becomes 0 Hz, the band width of the reception electronic circuit 106 becomes close to 3 times the signal band width.
Consequently, in the conventional coherent optical receiver to which the method of (III) is applied, an electronic circuit having 3 times or more the signal band width with respect to the signal light of for example 40 Gbit/s, that is to say a band width of 120 GHz or more is necessary, and for high speed signal light of 40 Gbit/s or more, realization of this is extremely difficult.